1. Field of the Invention
The present invention relates generally to a semiconductor device, and more particularly, to a semiconductor device with a MIS (Metal Insulator Semiconductor) capacitor.
2. Description of the Related Art
Conventionally, an MIS capacitor has been used in an electric device requiring large capacitance, such as an audio device.
FIG. 1 is a plane view showing a conventional MIS capacitor, and FIG. 2 is a cross-sectional view taken along line I--I in FIG. 1.
Referring to FIGS. 1 and 2, an N.sup.+ -type diffusion layer 14 is formed in a surface portion of a p-type semiconductor substrate 12. An n.sup.- -type epitaxial layer 16 is formed on the N.sup.+ -type diffusion layer 14. The epitaxial layer 16 is formed in an insular shape in each MIS capacitor, which is separated by p-type diffusion layer 18 (inter-element isolation) reaching the substrate 12. An n-type diffusion layer 20 serving as a diffused-region-side electrode of the MIS capacitor is formed in the n.sup.- -type epitaxial layer 16. The n-type diffusion layer 20 is connected to a diffused-region-side led-out electrode 24 through a contact hole 22. The led-out electrode 24 is connected to a terminal J.
An insulating layer (dielectric element 26 made of an oxide or a nitride is formed over the n-type diffusion layer 20. A metal-side electrode 28 of the MIS capacitor is formed on the insulating film 26. The electrode 28 is connected to a terminal M. The insulating film 26 interposed between the metal-side electrode 28 and the n-type diffusion layer 20 serves as a dielectric element of the MIS capacitor.
The insulating film 26 is sandwiched between the metal-side electrode 28 and the n-type electrode 20 (the diffused-region-side electrode), thereby forming the MIS capacitor. A notch 30 is formed in a predetermined location in the metal-side electrode 28. The led-out electrode 24 is formed in the notch 30. By virtue of the provision of the led-out electrode 24 in this manner, a predetermined semiconductor device can be formed in a given region with high efficiency.
In the above structure, the metal-side terminal M and the diffused-side terminal J ar simply connected to both end portions of the MIS capacitor. In this type of MIS capacitor, the thickness of the insulating (dielectric) film 26 is great, e.g. several-thousand .ANG.. Because of this, a sufficient withstand voltage of the MIS capacitor is obtained. However, with a recent development of a MIS capacitor with high capacitance, it has become necessary to reduce the thickness of the insulating film 26 to about 500 .ANG.. A MIS capacitor with a thin dielectric element has a low withstand voltage, e.g. 10 V to 50 V. Thus, if an abrupt high voltage, i.e. an external surge, is applied to either the metal-side electrode 28 or diffused-side led out electrode 24, an insulation between the metal side and the diffused side is easily broken by short-circuit.
In this case, a current path is formed in a region where the insulation has been broken, and the function of the capacitor is lost. It is difficult to expect when an external surge is applied to the capacitor. In some cases, the external surge is applied after the capacitor is built in a product or after the product is marketed. Thus, it is highly possible that defects are found in the product including a semiconductor device with such a MIS capacitor.